I came across this yesterday. I am trying to learn piano but tbh im not at a stage where i can play it as a game so i cant comment on how well it works realy except it seems to do what it says on the tin.
As the title suggests its Guitar hero but for piano, except you aint just randomly mashing buttons to the rhythm, you are actually playing piano. It works with a midi keyboard and doesnt need any special soundcard. You can choose any song you want so long as you have a midi for it.
The download seems to take ages to get started but once it does it downloads quick enough. Take a look
Synthesia, Piano for Everyone